I would have to agree with anyone who said it will probably start as a terrorist attack. There are literaly thousands of shipping containers that come into this country everyday and we don't have the time or money or manpower to search all of them. In fact only about 1% of them are searched.
That's one of the many reasons I prefer to live in the country and not a city. They won't waste a bomb on a rural area of farmers if they can use it on an urban area and kill millions.
Century Arms AR-15A2, 5.56
1946 Mosin-Nagant M44, 7.62x54R
1978 YUGO SKS, 7.62x39
NEF Protector Pump, 12 gauge
Savage Model 64, .22LR
NEF Pardner Tracker II, 12 gauge
Rock Island Armory 1911A1, .45 ACP
Remington 870 'Express Magnum' 12 gauge
Bersa Thunder .380